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The Rise of Erosion |
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This old standby of the old geology is back on the scientific cutting edge, starting with global climate.
Traditional geology, the kind your grandparents learned in school, is all about erosion. That's the all-encompassing name for the processes that turn rock into sediment and carry it from high elevations to low elevations. Erosion is responsible for the shape of the land around usnot just things like the Grand Canyon, but your own neighborhood. Erosion is what fills the rivers with silt and piles the beaches with sand. Erosion is the very reason that streams exist and why governments spend billions dredging mud from the world's shipping channels.
But today's geologists are exploring different problems these days: the global climate cycle, the processes of high-pressure metamorphism, the tectonic processes that affect crustal plates. Erosion might not feel like a modern, way-new subject for a contemporary Earth scientist. Nevertheless, erosion keeps coming up in new ways in these cutting-edge contexts. Let's look at the first one here.
The global climate depends to a large extent on the carbon cycle. I don't mean the short-term fluctuations that underlie today's climate-change debate, but changes on the scale of many millions of years. The Cenozoic Era of the last 65 million years has been marked by steady cooling and a steady lowering of the carbon dioxide level in the air. That greenhouse gas is removed from the air by green plants, most notably the floating microscopic plants, or phytoplankton, in the sea. As they die, their organic remains rain down upon the seafloor and are partly buried and partly recycled by seafloor life. (The
Ehux page is full of detail on this process.)Erosion affects this process in two ways. First, the more erosion there is on land, the faster seafloor carbon is buried. The great rivers draining the young ranges of the Himalaya, the Andes, and the American cordillera account for an enormous amount of carbon burial. Second, the more erosion there is on land, the more mineral nutrients enter the ocean through the world's rivers. Dissolved iron, calcium, and silica from eroded rocks are quickly taken up by phytoplankton, so erosion also serves directly as a fertilizer.
The cool climate of the Cenozoic, therefore, is reinforced by erosion. And the strongest, most effective type of erosion is done by glaciers. In fact, one theory is that glaciers are such potent eroders that they set the height limit for mountains over the whole planet. For that reason, current thinking is that mountain-building, which exposes rocks to erosion both ordinary and glacial, is the real engine of Cenozoic cooling. And mountain-building comes directly from the movements of crustal plates. That insight places erosion right in the thick of current research.
An article in the July 2001 Geology explores this question in a new direction, using the Andes range of South America as a natural laboratory that runs north-south through three different climate zones. The different types of erosion that occur in the trade-wind zone, the dry latitudes farther south, and the wet temperate zone south of that correlate strongly with the form of the mountains. Whereas most theorists have tried to explain these details of the Andes only through internal, tectonic mechanisms, the authors of this study(David Montgomery, Greg Balco and Sean Willett of the University of Washington) show that erosion must be in the mix too.